1. Field
The presently disclosed subject matter relates to a surface light source device and an LCD unit including the same, and more particularly to a surface light source device which can be configured to exhibit high brightness from both surfaces of the surface light source. Thus, the surface light source device can be employed as a light source for a back light unit located adjacent a liquid crystal display (LCD) for a personal computer and the like, in which the surface light source device can also illuminate a display located in an opposite direction of the LCD.
2. Description of the Related Art
Conventional optical transmission displays such as LCDs and the like are typically illuminated from a rear portion thereof by a surface light source device in order to light their displays. In a color LCD unit used for a personal computer or a television, the light transmission rate thereof decreases due to various reasons including restriction of an aperture ratio, use of a color filter, etc. Thus, the LCD unit is illuminated from its back using a back light unit like the surface light source device.
The surface light source device for a back light unit that illuminates from the back of the display unit can be broadly classified as an edge light type and a direct light type (when classified by location of the light source). The surface light source device of the direct light type is composed of a diffusing member and a plurality of light sources, which are located rearward of the diffusing member so as to emit a diffusing light from the plurality of light sources via the diffusing member. Thus, the direct light type device may illuminate an LCD with high brightness, but the LCD unit using the direct light type device may have a large thickness.
On the other hand, the edge light type is composed of a light guide and at least one light source, which is located at least one edge of the light guide so as to emit light from one surface of the light guide. Thus, an LCD unit using an edge light type device is thinner compared to an LCD unit using the direct light type. However such an LCD unit may be darker than that using the direct light type device because the LCD unit is indirectly illuminated via the light guide. Therefore, various edge light type surface light sources with high brightness have been devised.
For example, a surface light source with high brightness is disclosed in Patent Document No. 1 (Japanese Patent Application Laid Open H06-082628). FIG. 13 is a cross-section view depicting a basic structure for a conventional surface light source device, which is disclosed in Patent Document No. 1. The conventional surface light source device 51 includes: a light guide 53 having a first surface and a second surface; a diffusing sheet 55 located adjacent the first surface of the light guide 53; a reflecting sheet 56 located adjacent the second surface of the light guide 53; a plurality of discharge lamps 54 located adjacent a surface between the first and second surfaces of the light guide 53; and a reflector 54a located so as to cover the discharge lamps 54 therewith.
In the surface light source device 51, light emitted from the discharge lamps 54 enters into the light guide 53 and is diffused in the light guide 53 while reflecting on the reflecting sheet 56, and finally is emitted from the first surface of the light guide 53 through the diffusing sheet 55. In this case, the plurality of discharge lamps 54 is located adjacent the surface between the first and the second surfaces of the light guide 53, which includes a plurality of dot patterns on the second surface in order to provide a uniform light-emission. Therefore, the surface light source device 51 can improve the brightness while maintaining similar in thickness and in uniform-emission to an existing surface light source device.
A reflector for a surface light source device is disclosed in Patent Document No. 2 (Japanese Patent Application Laid Open JP2002-117715). According to Patent Document No. 2, light emitted from a lamp (corresponding to mark 54 in FIG. 13) can be reflected into a light guide (corresponding to mark 53 in FIG. 13) with a reflector (corresponding to mark 54a in FIG. 13) that is made by forming a silicone hard coated layer on a silver layer. Therefore, a surface light source device using the reflector may improve the brightness because the light emitted from the lamp may enter into the light guide with high efficiency.
A surface light source device with high brightness is disclosed in Patent Document No. 3 (Japanese Patent No. 3,688,398). According to Patent Document No. 3, a reflecting sheet (corresponding to mark 56 in FIG. 13) is composed of a light-diffusing reflective surface on an area located close to a light source (corresponding to mark 54 in FIG. 13) and a complete reflective surface on the other area. Therefore, the surface light source device using the reflecting sheet may improve the brightness because a light that is reached on the reflecting sheet can be reflected into the light guide with high efficiency.
Meanwhile, LEDs have been used as a light source for surface light source devices due to commercialization of high brightness LEDs. FIG. 11 is a partial cross-section view depicting a relevant part of a conventional surface light source device in which LEDs are used as a light source. The basic structure of the surface light source device 101 is similar to that of the surface light source device shown in FIG. 13, because the structure basically only replaces the discharge lamps in FIG. 13 with an LED light source.
The surface light source device 101 includes: a light guide 103 having a first surface and a second surface and a third surface 103a; a diffusing sheet 105 located on the first surface of the light guide 105; a reflecting sheet 106 located on the second surface of the light guide 103; the LED light source 104 located adjacent the third surface 103a of the light guide 103; and a casing 102 covering the LED light source 104 and the reflecting sheet 106 therewith.
The light guide 103 is composed of a flat transparent material such as an acrylic resin and the like. The first surface and the second surface are substantially parallel with respect to each other. The third surface 103a is an end surface between the first and second surfaces, and is substantially perpendicular to the first and the second surfaces. The diffusing sheet 105 is located on the first surface of the light guide 105, and the reflecting sheet 106 is located on the second surface of the light guide 105 as shown in FIG. 11.
The LED light source 104 is located adjacent the third surface 103a of the light guide 103, and the third surface 103a can operate as an incoming surface for light emitted from the LED light source 104 into the light guide 103. The LED light source 104 is composed of a plurality of LEDs, which are mounted on a flexible printed circuit board 107 via solders so as to align along the third surface 103a of the light guide 103. The flexible printed circuit board 107 that mounts the LED light source 104 thereon is attached to the casing 102 via a white adhesive tape 108.
A part of the reflecting sheet 106 is attached to the adhesive tape 108, and a part of the light guide 103 that mounts the reflecting sheet 106 on the second surface thereof is attached to the adhesive tape 108. Consequently, the LED light source 104, the printed circuit board 107, the light guide 103 and the reflecting sheet 106 can be fixed in the casing 102.
A white reflecting sheet, a metallic reflecting sheet and the like is used as the reflecting sheet 106. The white reflecting sheet is made by kneading an inorganic white pigment such as barium carbonate, barium sulfate and the like into white polyester, a white polycarbonate, etc. The white reflecting sheet can improve a white color by generating bubbles therein in the manufacturing process.
The white reflecting sheet cannot completely reflect light, but a part of light may leak through the white reflecting sheet. However, the leak light may be effectively utilized. FIG. 12 is a perspective view depicting an exemplary illumination of a personal computer cover using the conventional surface light source device shown in FIG. 11. A logo M of a personal computer cover 12 may be illumined by the leak light that leaks from the white reflecting sheet, and the commodity value may be improved by the utility of the leak light.
The above-referenced Patent Documents are listed below and are hereby incorporated with their English abstract in their entireties.
1. Patent Document No. 1: Japanese Patent Application Laid Open H06-082628
2. Patent Document No. 2: Japanese Patent Application Laid Open JP2002-117715
3. Patent Document No. 3: Japanese Patent No. 3,688,398
As described above, illumination for the logo may be carried out by a characteristic of the white reflecting sheet in which light may leak. The rear illumination is not necessarily controlled under an optimal condition. Therefore, the surface light source device may have the characteristic or problem of not emitting light with high light use efficiency and a high brightness. Even if the LED light source 104 is provided with the reflector that is disclosed in Patent Document No. 2, the light use efficiency for the surface light source device may not change, instead, the manufacturing process may become complex.
When the reflecting plate disclosed in Patent Document No. 3 is employed as the reflecting sheet, the reflecting plate is basically composed of a complete reflective surface in order to improve the brightness. Thus, an amount of light emitted from the first surface of the light guide 105 may increase, but the logo that is located adjacent the reflecting sheet may not be illuminated because light may not leak through the reflecting sheet.
The disclosed subject matter has been devised to consider the above and other problems and characteristics. Thus, embodiments of the disclosed subject matter can include a surface light source device that can efficiently emit light from the multiple surfaces thereof with high brightness and uniform brightness. In addition, the surface light source device can be maintained thin even when increasing the brightness thereof and when the emitting area thereof becomes large. The disclosed subject matter can also include an LCD unit using the surface light source device as described above. The LCD unit can be configured to increase the brightness of the display, and a second optical transmission displays that is located on an opposite side of the LCD can also be illuminated adequately for any purpose.